The present invention relates generally to planar inverted-F antennas, and more particularly to such an antenna with improved performance characteristics that is particularly suitable for use in wireless telephones.
The wireless communication industry has expanded rapidly and many different frequency bands have been implemented. A need exists for wireless devices that operate in multi-frequency bands. Dual band antennas have been used to meet this need, however, many dual band antennas use a dual feed which introduces difficulties into the feed system. Dual band antennas permit wireless handsets to operate in different networks that have different frequencies. There are more than three frequency bands used in the world for wireless communications. It is possible, but expensive to place multiple antennas on handsets and it also increases the complexity of the handset.
The use of wireless (cellular) telephones is very widespread. Not only has the size of wireless telephones decreased in the past few years, but the functional capabilities of such telephones have increased as well. Some of these wireless telephones are smaller than the palm of a user. In order to operate effectively and to deliver the needed functionability required of today's wireless technology, a useful and reliable antenna must be utilized. Planar inverted-F antennas, also known by the acronym “PIFA” have been popular and used in wireless devices such as handheld telephones because a PIFA has a low profile geometry and it does not extends out of the telephone as do most monopole stubby antennas used in current wireless handheld devices.
Notwithstanding the size advantages, many low profile antennas in use today have a narrow bandwidth. This parameter of bandwidth is limited in most applications by the need to match the impedance of the antenna to the system with which it is used. Conventional PIFAs, such as that described in U.S. Pat. No. 5,764,190, issued June 1998, have large resonant frequencies of 1.58 to about 1.78 GHz but with a bandwidth of about only 5% of the resonant frequency. This is usually referenced by a 2:1 VSWR into a 50 ohm load. This structure has its own disadvantages, one of which is that it utilizes the casing of the telephone handset as a ground plane and the other of which is that even with its low profile and capacitive feed, its achieved bandwidth is only about 5% at a VSWR (Voltage Standing Wave Ratio) of 2 or less.
A number of telephones are described in the literature in broadening bandwidth. These techniques include the use of a parasitic structure with a resonant frequency near that of the during antenna structure. Another is the use of a stacked microstrip patch antenna described in the articles “Broadband Air-Filled, Stacked U-Slot Shorted Patch Antenna” in Electronic Letters No. 35, Pages 515-517 (1999) or in “Design Probe-fed Stacked Patches” in IEEE Transactions on Antennas and Propagation, Vol. 47, No. 12, Pages 1780-1784, December 1999.
There are new frequencies of wireless communication proposed for a high end of frequencies in the 34 Hz range. This is known as the UMTS band and will increase the frequency bandwidth to about 23% to encompass the most used frequency bands.
A need therefore exists for a low profile antenna that has a greater bandwidth than 5% to utilize most, if not all of current and proposed wireless frequency bands, but which still maintains a desirable small size. The present invention is directed to a low profile antenna that overcomes the aforementioned disadvantages.